1. Field of the Invention
The present invention relates to an expansion valve for a refrigeration cycle controlling a refrigerant pressure at an outlet side of a gas cooler of a vapor compression type refrigeration cycle based on a refrigerant temperature at the outlet side of the gas cooler, more particularly relates to one suitable for a supercritical refrigeration cycle using a refrigerant in the supercritical region of carbon dioxide (CO2) etc.
2. Description of the Related Art
In general, as a vehicle air-conditioning system, use of a vapor compression type refrigeration cycle circulating CO2 as a refrigerant through a sealed circuit, as shown in FIG. 4, comprised of a compressor 1, gas cooler 2, expansion valve 3, evaporator 4, accumulator 5, etc. is known. Further, as shown in FIG. 1, a cycle comprised of the refrigeration cycle plus an internal heat exchanger 8 is also well known. As a mechanical type expansion valve used for such a vapor compression type refrigeration cycle, a pressure control valve such as shown in Japanese Patent Publication (A) No. 9-264622, Japanese Patent Publication (A) No. 2000-193347, and Japanese Patent Application No. 2005-006344 has been known in the past.
On the other hand, in a conventional cycle using HFC134a as a refrigerant, the amount of superheating of the refrigerant at the evaporator outlet is controlled by using an expansion valve such as shown in FIG. 11. To control the amount of superheating, the refrigerant temperature at the evaporator outlet has to be accurately detected. To accurately detect the refrigerant temperature, a type like the cassette type expansion valve shown in FIG. 11 where the temperature sensing part as a whole is arranged in the refrigerant passage is optimal. An example of this type of expansion valve applied to a CO2 refrigerant is disclosed in Japanese Patent Publication (A) No. 2000-193347.
However, with a CO2 refrigerant expansion valve, when detecting the temperature of the high pressure refrigerant at the gas cooler outlet, in the above system where the entire temperature sensing part is arranged in the refrigerant passage (FIG. 11), the sectional area of the refrigerant passage is large. Therefore, for strength, the thickness of the housing at the temperature sensing part increases and therefore the volume and weight of the valve system increase.
For this reason, it would seem that a valve system of a type using an outside feeler bulb (FIG. 2) and, further, a type detecting the temperature below the diaphragm from the refrigerant passing through the refrigerant passage in the housing (FIG. 3) would be advantageous in that the valve system would not be increased in volume or weight. However, if applying these types of valve systems to a CO2 refrigerant, the following problems would arise.
That is, in a conventional cycle using HFC134a as a refrigerant, the refrigerant sealed in the temperature sensing part or a space above the diaphragm for detecting the refrigerant temperature is used in a two-phase gas-liquid state. The temperature of the temperature sensing part is lower than the temperature in the engine compartment or inside the vehicle, so in the temperature sensing part, the refrigerant condenses and forms a liquid. In the two-phase gas-liquid state, the refrigerant pressure is determined by the saturation temperature (that is, the refrigerant liquid temperature), so the refrigerant pressure is determined by the refrigerant temperature of the temperature sensing part. For this reason, the pressure in the temperature sensing part will never be affected by the temperature at a location other than the temperature sensing part.
Compared with this, in a cycle using a CO2 refrigerant as a refrigerant, the refrigerant sealed in the temperature sensing part for detecting the refrigerant temperature is used in a supercritical state. For this reason, the refrigerant pressure is not determined by just the refrigerant temperature of the temperature sensing part. It is affected by the refrigerant temperature at locations other than the temperature sensing part, that is, space above the diaphragm affected by the outside air temperature or the refrigerant temperature in the capillary tube.
On the other hand, an expansion valve is designed based on the idea of opening and closing the valve based on the refrigerant temperature in the cycle detected by the temperature sensing part. Further, as a parameter corresponding to the temperature of the sealed refrigerant of the temperature sensing part corresponding to the refrigerant temperature in the cycle, the pressure of the sealed refrigerant, that is, the control pressure, is used.
This goes to say that in a cycle using a CO2 refrigerant as a refrigerant, the pressure of the sealed refrigerant used as the control pressure no longer matches with the refrigerant pressure corresponding to the sealed refrigerant temperature of the temperature sensing part. That is, the control temperature point of the expansion valve becomes offset and the control characteristics of the expansion valve deteriorate.